1. Field of the Invention
This invention relates generally to a non-contacting voltage metering apparatus for monitoring the electrical voltage of a test surface. More specifically, this invention relates to a non-contacting voltage metering apparatus having improved high frequency response.
2. Background Information
It is known in the art to measure the electrostatic field and potential of a test surface using a non-contacting voltage metering apparatus employing means for modulating the capacitance between the metering apparatus and a test surface to be measured. Such apparatus is shown, for example, in U.S. Pat. No. 3,611,127 to Vosteen and assigned to the assignee of the present invention.
While such metering apparatus is highly accurate and stable, the frequency range, or bandwidth, over which the metering apparatus may effectively operate is limited by the frequency at which the capacitance is modulated.
Typically, the alternating current (AC) potential of the surface under test may not be adequately measured for AC voltages in excess of one-half the frequency of modulation. For example, a conventional metering apparatus having a modulating frequency between 1000 and 2000 hertz, will have a maximum frequency response of between 500 and 1000 hertz.
Various techniques have been used in the prior art to increase the frequency range for voltage metering apparatus. For example, metering apparatus having somewhat higher frequency response is known which utilizes a preamplifier and an integrator having a high frequency response path therebetween for high frequency signals. However, the introduction of the modulating frequency signal, which is itself a signal of relatively high frequency, into the integrator limits long-term stability in the metering apparatus.
Accordingly, an object of the present invention is to provide an accurate, stable, non-contacting voltage metering apparatus for measuring the electrostatic field and potential of a test surface, wherein the metering apparatus has an improved high frequency response.
A further object of the invention is to provide a metering apparatus which has improved noise immunity.
Another object of the present invention is to provide a metering apparatus having improved high voltage measuring capability.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description or may be learned by practice of the invention.